The overall objective of the proposed research is to assess the feasibility of using metabolic tests to determine the viability of individual preimplantation ovine embryos. The first phase of the proposed research will use non-invasive, microfluorometric test to measure the uptake of glucose and pyruvate by ova and embryos up to the blastocyst stage. These data will be used to establish the types and rates of substrate usage during the preimplantation period. Embryo transfer experiments will then be carried out to determine the possible relationship between substrate uptake in vitro and the ability of embryos to develop in vitro. In the second phase of the proposed experiments an alternate test of metabolic state based on mitochondrial function will be used. Mitochondrial function will be assessed by measuring mitochondrial membrane potential using the cationic dye, rhodamine 123. Experiments will be carried out to examine the variation in staining between embryos and between blastomeres of individual embryos. Embryos will be transferred to determine if variability in embryo staining with rhodamine 123 is a good predictor of embryo viability. In the final phase of the research, embryo transfer experiments will be performed to determine if embryos selected on the basis of metabolic tests have a greater likelihood of developing to term than do embryos selected solely on the basis of morphological appearance. The proposed research has the potential to increase the efficiency of technologies using preimplantation livestock embryos by allowing only those embryos with a high probability of surviving to term to be transferred. The potential benefits of the research, however, are not limited to livestock. An objective test of embryo viability would be useful in helping to develop suitable culture conditions for embryos from endangered species as well as providing a means to assess embryos following in vitro fertilization and embryo storage in humans.